Generally, a cooling system for a liquid-cooled engine is provided with an apparatus for controlling coolant temperature in order to reduce engine friction and exhaust emissions. There are two common types of cooling systems. An exit-control cooling system controls coolant circulation using the temperature of coolant exiting an engine body. An entrance-control cooling system controls the temperature of coolant that flows into the engine body.
The exit-control cooling system has a simple structure, but in this system, coolant circulation cannot be precisely controlled. Recently, the entrance-control cooling system has been widely used for precise control of coolant circulation.
A conventional entrance-control cooling system typically includes a coolant bypass passageway, one end of which is connected to the engine body and the other to the thermostat. The bypass passageway provides coolant that has absorbed heat from the engine body to the thermostat. The main valve of the thermostat is thus opened or closed according to the temperature of the coolant provided through the coolant bypass passageway.
However, in such an entrance-control cooling system, an extra bypass valve and the coolant bypass passageway make the structure of the cooling system complicated. Furthermore, because some portion of the heated coolant in the engine body is bypassed through the coolant bypass passageway, the heating capacity of an associated heater can be decreased.